The present invention relates to a papermaking fabric for use in the paper manufacture.
In the conventional papermaking process carried out by using a fourdrinier wire screen, a slurry of raw paper material is fed onto an endless papermaking woven fabric or cloth running continuously. The papermaking cloth separates cellulosic fibers from the slurry to thereby form a moist paper web thereon. From this viewpoint, it can be said that the papermaking cloth or woven fabric functions as a filter for forming the moist paper web. Mesh apertures of the fabric also referred to as the drain orifices serve for separating water from the slurry. Further, in the case of a fourdrinier machine the papermaking fabric serves also as a driving belt and is thus subjected to a tension exerted by the machine. In view of this, it is required that the papermaking fabric should exhibit an enhanced stability in respect to the attitude.
Among the several requirements imposed on the papermaking process, the following are to be mentioned among others in connection with the papermaking fabric. Namely, the papermaking fabric is required to exhibit a high retention capability for the minimum flow loss, no generation or formation of wire marks, a high drainage capability with a reduced water containing capacity, a high capability of abrasion resistance, and enhanced running stability.
With a view to satisfying the above requirements imposed on the papermaking woven fabric, there have heretofore been made a variety of proposals. However, at the present state of the art, there are yet unavailable the papermaking woven fabrics which satisfy the abovementioned requirements to the satisfactory extent.
By way of example, the papermaking fabric woven finely by using fine yarns in an effort to enhance the raw paper material retention capability while preventing formation of the wire marks suffers from such shortcomings that the runability and the abrasion resistance capability were poor. In recent years, attempts have been made to form the papermaking surface of the fabric from the wefts for thereby improving the paper material retention capability. The papermaking surface formed from the wefts is certainly advantageous in that the woven fabric is improved in respect to the drainage property because of little or no possibility of the drain apertures formed between the warps being directly blocked by the raw paper material. In that case, it is however noted that the wire marks become more noticeable because the inter-weft gaps are increased correspondingly.
As an approach to solve the above problem, it has been proposed to increase the number of the wefts forming the papermaking surface by disposing so-called floating yarns which are not usually woven into the texture of the fabric in the form of interweave with the warps and the wefts. This proposal is certainly an interesting technical idea from the standpoint of increasing the number of the wefts of the papermaking surface of the fabric, which idea cannot however be applied to practical papermaking process. The reason for this is that the wefts not woven into the texture, i.e. the floating yarns tend to be moved and collected together under a hydraulic pressure applied thereto upon charging of the slurry on the papermaking fabric, resulting in that the papermaking surface cannot be maintained horizontally flat or smoothly.
The problem of formation of the wire marks becomes more noticeable in the case of the single-layer woven fabric in which the wefts form projections on the papermaking surface.
There has also been proposed the use of a multi-layer woven fabric in an effort to realize a high drainage property and a papermaking surface of a fine mesh while ensuring a high abrasion resistance capability.
Recently, there has arisen a trend that the papermaking process be carried out at a higher speed with a view to increasing the efficiency of paper manufacture process, which however presents additional new problems. The multi-layer woven fabric which can certainly exhibit advantageous effects unattainable with the single-layer woven fabric has a high water retaining property which is primarily ascribable to the multi-layer structure. By way of example, when the endless screen formed of the multi-layer woven fabric is driven at a high speed, there will take place such a phenomenon that water droplets are caused to fly out particularly at positions of the rotating turn-back rolls under a centrifugal force.
In this conjunction, it is noted that the single-layer woven fabric is substantially insusceptible to the phenomenon mentioned above due to inherently small water retention capacity. However, the single-layer woven fabric is conventionally provided with long knuckles disposed over the running surface for ensuring the abrasion resistance capability. Consequently, the papermaking surface assumes such a configuration that the long knuckles of warps are disposed in parallel with the knuckles of the weft projecting therebetween. When the raw paper material slurry is supplied onto the papermaking fabric or clothing in the course of running thereof, the paper fibers are necessarily oriented in the running or machine direction to be deposited between the long knuckles of the warps disposed in parallel on the cloth or caused to displace downwardly. The deposition or accumulation of the fibers between the parallel long knuckles of the warps will necessarily block the flow paths through which water can be drained, resulting in an obstacle to the desired drainage. In order to avoid this difficulty, the vacuum must be increased, which will however favor the appearance of the wire marks.
In the course of intensive studies conducted by the inventor of the present application in tackling the solution of the problems associated with the requirements for a high paper material retention capability, suppression of generation of the wire marks, a high water drainage capability and a low water containing capacity, a high abrasion resistance capability and an improved runability and others, it has been found that an increased density of the wefts in the papermaking surface of the fabric is not suited for improving the raw paper material retention capability, while the multi-layer woven fabric is subjected to a limitation in reducing the water retention capacity of the fabric, requiring thus the use of the single-layer woven fabric, and that not only the plain weave but also the twill weave and satin weave of the single-layer fabric cannot reduce the void volume formed by the warps, being ineffective in preventing the formation of the pulp mats between the warps.